Identification: DNA typing of degraded samples: practice question

A Forensic Medicine question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Forensic Medicine · Identification: DNA typing of degraded samples

Sample item

Bones recovered from a house fire yield only highly fragmented DNA. A reference sample from a parent is available. Which technique has the best chance of giving an individualising profile?

AABO grouping of bone extractBShort tandem repeat typing using reduced-size amplicons (mini-STR)CRestriction fragment length polymorphism with Southern blottingDKaryotyping of bone marrow cells

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

B. Short tandem repeat typing using reduced-size amplicons (mini-STR)

Mini-STR kits place primers close to the repeat and amplify small products, so they work on degraded DNA that has broken into short pieces. RFLP needs large amounts of intact high molecular weight DNA, which fire and decay destroy. Parent-child comparison of STR alleles can then support identity.

Option by option

A. ABO grouping of bone extract
ABO grouping is not individualising and rarely survives in burnt bone.
B. Short tandem repeat typing using reduced-size amplicons (mini-STR) (correct)
Correct. PCR of short amplicons survives degradation and allows comparison with a parental reference.
C. Restriction fragment length polymorphism with Southern blotting
RFLP requires intact, high molecular weight DNA, which is lacking.
D. Karyotyping of bone marrow cells
Karyotyping needs living dividing cells, which are absent.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

Questions about this Forensic Medicine question

What is the answer to this Forensic Medicine question?

The best answer is B, Short tandem repeat typing using reduced-size amplicons (mini-STR). Mini-STR kits place primers close to the repeat and amplify small products, so they work on degraded DNA that has broken into short pieces.

Why is "ABO grouping of bone extract" not the answer?

ABO grouping is not individualising and rarely survives in burnt bone.

Why is "Restriction fragment length polymorphism with Southern blotting" not the answer?

RFLP requires intact, high molecular weight DNA, which is lacking.

Why is "Karyotyping of bone marrow cells" not the answer?

Karyotyping needs living dividing cells, which are absent.

What concept does this question test?

It tests Identification: DNA typing of degraded samples in Forensic Medicine. RFLP needs large amounts of intact high molecular weight DNA, which fire and decay destroy. Parent-child comparison of STR alleles can then support identity.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Identification: DNA typing of degraded samples?

Learn postmortem changes with their timelines, and toxicology as poison, sign, test and antidote. Then use the flashcards and the other Forensic Medicine questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.